Prime Minister Tony Abbott says all that is required for a domestic Islamic State ordered terror attack is a "knife, iPhone and a victim" but has urged Australians to go about their lives "insouciantly".

Mr Abbott said on Friday that Thursday's raids were a "show of strength" from authorities whom he said would be given all the resources and powers they need to disrupt further plots.

"Terrorists want to scare us out of being ourselves and our best response is to insouciantly be fully Australian, to defy the terrorists by going about our normal business," he told reporters in Sydney.

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An Australian, believed to be Mohammad Baryalei, a senior figure in Islamic State (also known as ISIL) in Syria, has been sending "strong orders" to his "small networks" in Australia to carry out "demonstration executions", Mr Abbott said.

Earlier, Mr Abbott said the attack would have occurred within days, if authorities had not acted.

"The advice of our police and security agencies was that attacks of this nature could take place within days, that is why they acted," he said.

Authorities say one of those orders was issued earlier this week.

"It is a serious situation when all you need to do to carry out a terrorist attacks is to have a knife, an iPhone and a victim," the Prime Minister said.

Mr Abbott cut short his visit to Arnhem Land to farewell troops leaving for Iraq. He convened a meeting of cabinet's National Security Committee and received intelligence briefings from the spy agency ASIO and the AFP.

Mr Abbott praised the authorities for their largest counter-terrorism operation ever, involving 800 officers, and said Australians should be reassured by the agencies' ability to detect and respond to threats.

"The community expects government at all levels to keep them safe, this government will not let them down," he vowed.

AFP acting commissioner Andrew Colvin would not confirm how many of the 15 people arrested on Thursday have subsequently been released.

"There are limitations with what I'm prepared to say, both legislatively and operationally," he said on Friday.

Mr Abbott said the "vast majority" of Australians are "absolutely" on the side of the police and security agencies.

"The last thing any of us would want to do is to damage our freedoms in order to preserve our freedoms. But I don't think any reasonable observer would say that Australian police, that NSW police, that our security services are unreasonable or over the top," he said.

"We are simply doing what is necessary in difficult circumstances to keep our citizens safe."

Attorney-General George Brandis said on Friday that there would have been a "terrorist outrage on the streets of Sydney" had the agencies not had the powers to detect and foil the alleged plot.

"They picked this up, they were onto it, they interdicted it and disrupted it when they needed to, had they not had that skill and capability there would have been in all likelihood a terrorist outrage on the streets of Sydney in coming days," the Attorney-General told the ABC.

Senator Brandis said it showed why boosted powers, to be introduced into Parliament next week, were necessary.

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"We need to ensure that the police and intelligence agencies have all the powers that they need, subject of course to safeguards and parliamentary oversight to keep our community safe, that is what the government is absolutely determined to do," he said.

Latika Bourke is a national political reporter for The Sydney Morning Herald and The Age. She has previously worked for ABC and 2UE in Canberra. Latika won the Walkley Award for Young Australian Journalist of the Year in 2010.